Polycarbonate is a popular material for making ophthalmic lenses. Polycarbonate lenses are often produced by injection and compression molding of liquid resin. The conventional operation causes the lenses to remain with residual stress effects.
These residual stress effects can affect the dimensional stability of the lenses, and lead to other problems. For example, overly-stressed polycarbonate lenses can warp upon storage or wear. The edges of such lenses can have varied optical powers. A phenomena known as cold creep can also cause problems in the lenses.
The presently-described system teaches a new method and apparatus for forming low stress polycarbonate lenses.
The machine and process that is described herein produces finished ophthalmic lenses by simultaneously molding both the concave and convex surfaces of these lenses under controlled pressure and even heating.
A preferred mode presses both top and bottom at the same time, and at substantially the same pressures.
Another mode uses a mold made of glass to match the coefficient of expansion of the mold more closely to the polycarbonate blank. The mold is heated and cooled using a microgranulated solid that is pressed against the mold. This provides even heating and heat spreading.